Evaporation-assisted magnetic separation of rare earth ions in aqueous solutions

This work aims to answer the question of why an enrichment of paramagnetic ions can be observed in a magnetic field gradient [1] despite the presence of a counteracting Brownian motion. For that purpose, we study a rare earth chloride (DyCl3) solution in which weak evaporation is adjusted by means of small differences in the vapor pressure in a specially developed cell, see Fig. 1.
The temporal evolution of the refractive index field of this solution, as a result of heat and mass transfer, is measured by means of a Mach-Zehnder interferometer. We develop a numerical algorithm which splits the refractive index field into two parts, one space-dependent and conservative and the other time-dependent and transient. By using this algorithm in conjunction with a numerical simulation of the temperature and concentration field, we are able to show that 90% of the refractive index in the evaporation-driven boundary layer is caused by an increase in the concentration of Dy(III) ions. A simplified analysis of the gravitational and magnetic forces, entering the Rayleigh number, leads to a diagram of the system's instability. Accordingly, the enrichment layer of elevated Dy(III) concentration is placed in a spatial zone dominated by a field gradient force. This leads to the unconditional stability of this layer in the present field. The underlying mechanism is the levitation and reshaping of the evaporation-driven boundary layer by the magnetic field gradient [2].

[1] X. Yang, K. Tschulik, M. Uhlemann, S. Odenbach, K. Eckert, J. Phys. Chem. Lett. 3 (2012), 3559–3564.
[2] Z. Lei, B. Fritzsche, K. Eckert, submitted to J. Phys. Chem. C (2017).